The invention pertains to a cutter bit for use in conjunction with excavation equipment. More particularly, the invention pertains to a rotatable concave cutter bit for use in conjunction with excavation equipment such as, for example, a longwall shearer, a continuous mining machine, a trencher, a road milling machine, an auger and a saw.
Some conventional cutter bits used for excavation equipment utilize a single cutting element at the forward end thereof. In this particular application, it is only this single cutting element that forms the effective cutting element of the cutter bit that impinges upon and cuts or fractures the substrate such as, for example, earth strata. The balance of the forward end of the cutter bit pushes fractured or cut material out of the path of the cutter bit.
Another style of cutter bit for use with excavation equipment is a concave cutter bit. The typical concave cutter bit has an enlarged diameter portion, which contains a concavity, at the forward end thereof. A cutter element of hard material such as, for example, cemented tungsten carbide, surrounds the outer periphery of the concavity so that the cutter element presents a generally circular or ring-like shape. One example of a concave cutter bit is illustrated by U.S. Pat. No. 5,078,219 to Morrell et al. Another example of such a cutter bit is shown by U.S. Pat. No. 5,333,938 to Gale.
The cutter element can take the form of a single piece ring such as is shown by the Morrell et al. patent. Typically, the cutter element is made out of cobalt-cemented tungsten carbide and the bit body is made from steel. The cutter element is secured to the steel bit body by brazing so that, at a minimum, there is a braze joint between the bottom surface of the carbide cutter element and the surface of the cutter bit body.
Carbides such as cobalt-cemented tungsten carbide have coefficients of thermal expansion that are approximately one-half to one-third that of steel. Because of this difference in thermal expansion, the steel bit body and the cemented tungsten carbide cutter element contract at different rates upon cooling after the brazing operation. This difference in contraction creates cracks in the braze joint and/or brazing stresses at the braze joint.
The existence of cracks in the braze joint can cause the bit to fail quality control inspection and be discarded as scrap. The existence of cracks or brazing stresses can lead to the early failure of the concave cutter bit during operation. It is apparent that the failure of the cutter bit to either pass quality control examination or function well in the field is undesirable.
The cutter elements can also take the form of a plurality of segments positioned adjacent to one another in an end-to-end relationship so as to form a complete ring. It has been found, however, that the presence of cracks and braze stresses are not reduced by the use of a plurality of cutter insert segments in comparison to a cutter bit with a single piece ring-shaped cutter element. For those cutter bits where the cutter element comprises a plurality of segments, each segment is positioned so that its end surfaces are near, but slightly spaced apart from, the corresponding end surface of the adjacent cutter element. In the past, the distance of the spacing has been about 0.020 inches.
During the brazing operation, braze alloy flows between the opposite ends of adjacent cutter element segments to form a continuous volume of braze alloy between the opposite end surfaces of the adjacent cutter element segments. A volume of braze alloy also exists between each one of the cemented tungsten carbide cutter element segments and the steel cutter bit body.
Upon initial cooling after the brazing operation, the braze joint between the opposite end surfaces of adjacent cutter element segments solidifies as does the braze joint between the cutter element segments and the cutter bit body. At this point in time, however, the steel cutter bit body and the cutter element segments must still cool to room temperature.
As the cutter bit and cutter element segments continue to cool and contract, the difference in the rate of contraction between the steel bit body and the cutter element segments, which now behave as if they were one piece, creates braze stresses or cracks in a fashion like that for the single piece cutter element.
It thus becomes apparent that the problems associated with brazing stresses and braze joint cracks exist for concave cutter bits having either a single piece ring-shaped cutter element or a cutter element comprising a plurality of segments where a continuous braze joint forms between the opposing end surfaces of the adjacent segments.
Thus, it would be desirable to provide an improved concave cutter bit that does not experience, or at least has reduced, brazing stresses and brazing cracks. As a consequence, such a concave cutter bit would experience less quality control rejections, as well as fewer premature failures so as to provide a longer, more consistent useful life.